Such machines are already known. They have on their lower part two milling drums with parallel horizontal axes and tools on their periphery which are made to rotate in opposite directions and thus cut trenches by progressively loosening the soil and gathering between them the resulting materials which are then aspirated by a pump.
According to known design, each drum is made of two semi-drums supported on either side of a thin, flat, metallic part which is vertical and perpendicular to the drum's axis.
Since the weight of the machine is what makes it sink into the ground as the soil is loosened by the tools located on the drums, it is of utmost importance, when working in rocky soil, that the central part be as thin or narrow as possible because the tools cannot loosen soil located under this central part.
Furthermore, since the operating drums are subjected to very high stress and sizable vibrations, the typical hydraulic motors whose frames are fastened to the central part and whose central shafts support the semi-drums are subjected to such strain that they have a rather short life expectancy.